While virtual memory makes it possible for computers to more easily
handle larger and more complex applications, as with any powerful tool,
it comes at a price. The price in this case is one of performance
— a virtual memory operating system has a lot more to do than an
operating system incapable of supporting virtual memory. This means
that performance is never as good with virtual memory as it is when the
same application is 100% memory-resident.
However, this is no reason to throw up one's hands and give up. The
benefits of virtual memory are too great to do that. And, with a bit of
effort, good performance is possible. The thing that must be done is to
examine those system resources impacted by heavy use of the virtual
memory subsystem.
For a moment, take what you have read in this chapter and consider
what system resources are used by extremely heavy page fault and
swapping activity:
RAM — It stands to reason that available RAM is low
(otherwise there would be no need to page fault or swap).
Disk — While disk space might not be impacted, I/O
bandwidth (due to heavy paging and swapping) would be.
CPU — The CPU is expending cycles doing the processing
required to support memory management and setting up the necessary
I/O operations for paging and swapping.
The interrelated nature of these loads makes it easy to understand
how resource shortages can lead to severe performance problems.
All it takes is a system with too little RAM, heavy page fault
activity, and a system running near its limit in terms of CPU or disk
I/O. At this point, the system is thrashing, with poor performance
the inevitable result.
At best, the overhead from virtual memory support presents a
minimal additional load to a well-configured system:
RAM — Sufficient RAM for all working sets with enough
left over to handle any page faults
Disk — Because of the limited page fault activity, disk I/O
bandwidth would be minimally impacted
CPU — The majority of CPU cycles are dedicated to
actually running applications, instead of running the operating
system's memory management code
From this, the overall point to keep in mind is that the
performance impact of virtual memory is minimal when it is used as
little as possible. This means the primary determinant of good
virtual memory subsystem performance is having enough RAM.
Next in line (but much lower in relative importance) are
sufficient disk I/O and CPU capacity. However, keep in mind that
these resources only help the system performance degrade more
gracefully from heavy faulting and swapping; they do little to help
the virtual memory subsystem performance (although they obviously can
play a major role in overall system performance).
